The present invention relates to a system for using signals scattered by one or more targets to determine position and velocity for each of the targets. The targets are situated in a position space comprising a number of transmitters and receivers of electromagnetic or acoustic signals, these transmitters and receivers being dispersed to a number of known points in the position space. The system will be discussed in the following with respect to a radar application. The invention is however equally suited for use in acoustic systems and it is the stated intention of the applicant that this patent application shall concern such systems as well. The system can in general be used in the case where the positions in space and velocity vectors for a very large number of targets shall be determined.
Radar systems of today for surveillance and combat command consist of a small number of long range and capable radar stations. These systems are very vulnerable, partly because the radar stations are relatively easy to detect and partly because they are so few in number. The object of the invention is to improve this situation. The improvement consists partly in that the surveillance function is distributed over a large number of dispersed but relatively simple radar stations, where some can be lost without the position picture being impaired in a crucial manner. As well, single radar stations are small and easily transported and can be put in place on short notice without comprehensive ground installations or other preparations and therefore cannot be detected in advance.
Further motivation to the need of the present new radar technique is that military aircraft of coming generations are expected to have a smaller and smaller radar cross section. Already today there are so-called stealth aircraft which have radar cross sections less than a ten-thousandth of conventional aircraft. In the near future new types of military aircraft will be operative which have just as small radar cross sections but which as well are capable of high performance with respect to velocity and maneuverability. These aircraft will at first be few in number but in time the technique will likely spread to many types of aircraft and to the air force of many nations.
Fundamental physical reasons limit the possibility of stealth embodiment. The ideal, stealth aircraft (something which cannot be constructed today and maybe not even in the future) has a complete electromagnetic adaptation to the surrounding airspace so that incoming radar radiation is completely absorbed by the aircraft. This implies that no reflection (or in other words backscattering) occurs in the direction −{right arrow over (N)} regardless of the direction {right arrow over (N)} for the incoming radiation. However in the physics of scattering it applies that even in this case the aircraft has a non-disappearing scattering cross section in the directions {right arrow over (N)}′ which distinguish themselves from the backscattering direction −{right arrow over (N)}. In fact the scattering cross section in the extended direction {right arrow over (N)} for the illuminating radiation is independent of electromagnetic adaptation and stealth embodiment and makes up the square of the geometric cross section of the aircraft projected in the direction {right arrow over (N)} divided by the square of the wavelength. Around the energy-scattering concentrated in {right arrow over (N)} and at angles which can very well be close to 90° in relation to {right arrow over (N)}, scattering cross section with suitable choice of wavelength can be expected in parity with the prevailing conventional aircraft of today.
As shall be evident in detail below, the present proposal is based on so-called bi-static radar geometries. In that these are combined with relatively low radar frequencies (UHF) the above-mentioned easily measurable scattering cross sections for stealth aircraft are achieved, and the aircraft can thereby be detected.